Allelic variation at 13 sites for the intertidalPomatoceros lamarckii(Quatrefages) and at six sites for the sublittoralP. triqueter(L.) in North‐West Europe, when examined at 19 metabolic enzyme loci, showed significant differences between sites in the frequencies of the various enzyme morphs. There was no readily interpretable geographic or environmental trend. The Hardy‐Weinberg equilibrium was maintained at almost all loci and all sites. The genetic distance between populations was very small in comparison with the difference between the two species.Colour morphs ofP. triquetershowed a clear latitudinal cline with blue and brown morphs predominating at more northerly latitudes, and grey, orange and red morphs at more southerly latitudes.Interlocality variations in allele frequency, though statistically significant, were never sufficient to alter the order of frequency of the two or three commonest alleles, so that the frequency of all but rare alleles showed a consensus throughout the ranges studied for each species. Accounting for local genetic patchiness in gene frequencies in species with pelagic larvae raises problems if the alleles are selectively neutral; accounting for wide geographic uniformity raises problems if alleles are selected for or against by environmental fact